1. Field of the Invention
The present invention relates to the field of plasma assisted deposition and etching of a workpiece using a high density plasma source.
2. Description of the Related Art
Plasma reactors, particularly radio frequency (RF) plasma reactors of the type employed in semiconductor wafer plasma processing in the manufacturing of microelectronic integrated circuits, confine a plasma over a semiconductor wafer in the processing chamber by walls defining a processing chamber. Such an approach for plasma confinement has several inherent problems where employed in plasma reactors for processing semiconductor wafers.
First, the walls confining the plasma are subject to attack from ions in the plasma, typically, for example, by ion bombardment. Such attack can consume the material in the walls or introduce incompatible material from the chamber walls into the plasma process carried out on the wafer, thereby contaminating the process. Such incompatible material may be either the material of the chamber wall itself or may be material (e.g., polymer) previously deposited on the chamber walls during plasma processing, which can flake off or be sputtered off. As one example, if the chamber walls are aluminum and the plasma process to be performed is plasma etching of silicon dioxide, then the material of the chamber wall itself, if sputtered into the plasma, is incompatible with the process and can destroy the integrity of the process.
Second, it is necessary to provide certain openings in the chamber walls and, unfortunately, plasma tends to leak or flow from the chamber through these openings. Such leakage can reduce plasma density near the openings, thereby upsetting the plasma process carried out on the wafer. Also, such leakage can permit the plasma to attack surfaces outside of the chamber interior. As one example of an opening through which plasma can leak from the chamber, a wafer slit valve is conventionally provided in the chamber side wall for inserting the wafer into the chamber and withdrawing the wafer from the chamber. The slit valve must be unobstructed to permit efficient wafer ingress and egress. As another example, a pumping annulus is typically provided, the pumping annulus being an annular volume below the wafer pedestal coupled to a vacuum pump for maintaining a desired chamber pressure. The chamber is coupled to the pumping annulus through a gap between the wafer pedestal periphery and the chamber side wall. The flow of plasma into the pumping annulus permits the plasma to attack the interior surfaces or walls of the pumping annulus. This flow must be unobstructed in order for the vacuum pump to efficiently control the chamber pressure, and therefore the pedestal-to-side wall gap must be free of obstructions.
The above problems are intensified in reactors capable of generating high density plasmas such as a Magnetically Enhanced Inductively Coupled Plasma reactor. With the Magnetically Enhanced Inductively Coupled Plasma reactor or MEICP reactor, the plasma is generated by generating a helicon wave in a plasma source chamber located above and in an axial relationship to a substrate processing chamber. The substrate processing chamber typically is surrounded by vertical permanent magnets extending part way down the outside of the processing chamber to form a magnetic bucket for confining the plasma as it enters from the source chamber.
With this type of MEICP reactor, the permanent magnets are oriented so that one pole of each magnet faces the interior of the processing chamber with successive magnets having an opposite polarity facing the chamber. This creates a magnetic field with vertical cusps, which extend part way into the interior of the processing chamber.
It is significant to note that the permanent magnets extend only part way down the sidewall of the processing chamber. A portion of the chamber is located below the magnetic bucket, including the workpiece insertion opening and the pump opening.
With this type reactor, the magnetic bucket is used to provide a uniform plasma in a central zone within the processing chamber. The workpiece is placed on a pedestal below the magnetic bucket and then raised into the magnetic bucket for processing.
This type of plasma reactor also utilizes bias power and cooling apparatus connected via lines to the pedestal. As the pedestal must be moved into the magnetic bucket, the bias power and cooling lines must follow which is not desirable. Also, the pedestal adjustment mechanism itself must be maintained to ensure proper functionality.
As discussed above, a problem inherent to plasma reactors, including MEICP reactores, is degradation of reactor components exposed to the plasma. The surface of the walls confining the plasma are subject to attack from ions in the plasma. Such attack can consume the material causing periodic maintenance and reactor down time. In addition, the chamber wall may introduce an incompatible material into the plasma. For example, if a silicon dioxide etch is being performed and the chamber walls are aluminum, the aluminum may sputter into the plasma and deposit on the workpiece, thereby contaminating the workpiece. Likewise, any by-products deposited on the chamber walls may also sputter or flake off and contaminate the workpiece.
In addition to the surface of the walls, any openings, such as the opening for workpiece placement on the pedestal or the vacuum pump opening are also subject to degradation. Such degradation may prevent those openings from functioning properly. For example, the workpiece opening which is normally sealed during processing may not seal properly due to degradation and as a result, allow air to leak in thereby reducing plasma density and upsetting workpiece processing. Or, by-products may deposit in or around the opening prohibiting insertion of the workpiece. Likewise, the pumping opening may become obstructed thereby affecting pumping efficiency.
Another problem with this type reactor is that the magnetic bucket protects the sidewalls but not the top wall. The top wall or ceiling, therefore, may be exposed to plasma and other process gases. Because the plasma near the ceiling has very high density as it exits the plasma source chamber, the exposed ceiling experiences a greater degree of degradation by the plasma.
Furthermore, because the ceiling is unprotected, build-up is more likely to occur. Build-up on the ceiling can change processing parameters and degrade processing. For example, to control ion energy at the workpiece, an RF bias may be applied to the workpiece, with the walls of the processing chamber serving as the anode and being grounded. In such a case, the coupling of bias power to the workpiece affects the ion energy. Any decrease in the bias power will reduce ion energy. Changes in bias power can occur, for example, when polymer is used during oxide etch to control etch selectivity. Polymer builds up on the anode and thereby changes its impedance. Polymer build-up on the anode, therefore, will cause ion energy to drift, thereby reducing ion energy at the workpiece.
Changes in ion energy will affect processing parameters. Changes in ion energy due to polymer build-up could be compensated for by increasing bias power during processing if the changes were predictable. Increasing bias power during processing, however, would further complicate processing and introduce uncertainties to processing.
Furthermore, the problem is exaggerated after several workpieces have been processed and more build-up is added to the anode. Cleaning of the chamber after processing each workpiece is possible but not desirable because it would lower throughput and increase costs.
In certain ion driven processes, such as oxide etch, ion energy is critical. Changes in ion energy during processing results in different and often unacceptable etch characteristics. For example, polymer build-up during an etch process could lead to etch stopping. Therefore, the build-up of polymer during ion etch is particularly problematic.
The present invention provides a plasma reactor having a plasma source chamber capable of generating a high density plasma. Typically, a magnetically enhanced inductively coupled source power applicator capable of generating a helicon wave is used to generate the plasma. The plasma source chamber is coupled to a processing chamber to allow the plasma to flow into the processing chamber, which has a pedestal for supporting a workpiece.
The plasma reactor of the present invention improves workpiece processing. Using a helicon wave to generate the plasma allows a uniform high density plasma to be generated in the plasma source chamber over a large range of temperatures and pressures. This allows the present invention to be employed over a large process window for both etching and deposition processes.
The plasma chamber source power applicator is typically a double loop antenna disposed around a bell chamber which is provided with current by an RF source generator so as to create a helicon wave. Although any type helicon wave may be used, an m=0 helicon wave is presently preferred. The plasma chamber power applicator may also include nested electromagnets which provide an axial magnetic field within the plasma chamber. The interaction between the axial magnetic field and the induced RF electric field within the bell chamber gives rise to the helicon wave and assists in directing its flow toward the processing chamber.
An advantage of higher density plasma is that plasma generation may be moved farther from the workpiece while still maintaining sufficient plasma density to process the workpiece. An advantage of moving plasma generation farther from the workpiece is that it allows the plasma to xe2x80x9ccoolxe2x80x9d before it reaches the workpiece. Allowing ions, particularly electrons, to xe2x80x9ccoolxe2x80x9d before impacting the workpiece reduces uneven charge build-up and resulting damage. Furthermore, the more uniform plasma prevents plasma nulls which are typical of conventional inductively coupled reactors and which cause uneven workpiece processing.
With conventional inductively coupled plasma reactors, the source power-to-workpiece gap is much smaller than is allowed by magnetically enhanced reactors. A larger gap would cause the power density drops which would unacceptably lower the etch rate. The magnetically enhanced inductively coupled source power applicator, on the other hand, provides more efficient coupling of source power to the process gases, and so it is able to provide nearly 100% ionization in the plasma. This allows the gap to be increased while maintaining sufficient plasma density for processing.
This also allows lower operating pressures which provides a larger plasma operating window. The source power applicator of the present invention, therefore, allows the present invention to be run at lower pressures. As higher pressure narrows the process window by causing faster recombination of plasma particles, lower operating pressure expands the operating window. The magnetically enhanced source power applicator of the present invention allows the present invention to be operated over a wide range of RF source power, magnetic field strength, and pressures, thereby expanding the processing window.
While high density plasma generally improves processing, it may also increase reactor component degradation. Components such as the workpiece insertion opening and associated mechanisms, the vacuum pump opening and associated mechanisms, and the processing chamber walls, among other components, are susceptible to damage by the plasma. To prevent plasma from flowing into the workpiece insertion opening, one embodiment of the present invention provides a magnetic field across a workpiece insertion opening of sufficient magnetic strength to inhibit plasma from advancing into the workpiece insertion opening from the processing chamber. This may be accomplished by employing a pair of magnets on either side of the opening. The magnets may have poles facing an axis perpendicular to the plane of the workpiece, or facing parallel to the axis, and may form an open magnetic circuit or a closed magnetic circuit.
As the workpiece insertion opening typically is horizontally located in the chamber wall, horizontal magnets may be located above and below the opening to provide the magnetic field across the opening. The horizontal magnets may annular and disposed around an axis perpendicular to the plane of the workpiece. Additional magnets may be provided above those located adjacent the workpiece insertion opening so as to create a cusped magnetic field that extends along the side wall of the processing chamber to form a magnetic bucket.
Another embodiment of the present invention provides a magnetic field across the vacuum pump opening of sufficient magnetic strength to inhibit plasma from advancing into the vacuum pump opening from the processing chamber. This may be accomplished by employing a pair of magnets on either side of the vacuum pump opening. The magnets may have poles facing an axis perpendicular to the plane of the workpiece, or facing parallel to it, and may form an open magnetic circuit or a closed magnetic circuit.
In one embodiment, the vacuum pump opening is an annular opening located in the bottom wall of the processing chamber adjacent the side wall. With this embodiment, annular magnets are located on either side of the vacuum pump opening and may form a closed magnetic circuit, or an open magnetic circuit. Additional magnets may be added above one of the magnets, which may be located in, or adjacent the side wall, so as to create the cusped magnetic field that extends along the side wall of the processing chamber to form the magnetic bucket.
In another embodiment, the vacuum pump opening is located in the side wall of the processing chamber. In this embodiment, a portion of the vacuum pump opening may form the workpiece insertion opening. Magnets are located above and below the vacuum pump opening so as to inhibit plasma from entering the vacuum pump opening. With this embodiment, the pole axis of the magnets may face toward the interior of the processing chamber, or may face parallel to the walls of the processing chamber in an open magnetic circuit configuration. Additional magnets may be added above the upper magnet, so as to create the cusped magnetic field extending along the side wall of the processing chamber forming the magnetic bucket.
In yet another embodiment, the present invention provides a plurality of magnets disposed around an axis perpendicular to the plane of the workpiece, which form a magnetic bucket that extends the length of the side wall of the processing chamber. One advantage is that the pedestal need not be raised to be within the bucket. The magnetic bucket confines the plasma to improve plasma density over the workpiece and also confines the plasma away from reactor components, such as the lower portion of the processing chamber side wall. As discussed above, it may also confine plasma away from the workpiece insertion opening and the vacuum pump opening, among other components. In such a case, the present invention inhibits plasma from flowing into the workpiece insertion opening and the vacuum pump opening when a portion of the vacuum pump opening forms the workpiece insertion opening, or when the vacuum pump opening and the workpiece insertion opening are separate openings.
The magnetic bucket may be formed by permanent magnets oriented so that one pole of each of the plurality of magnets faces the interior of the processing chamber with adjacent magnets having opposite poles facing the interior of the processing chamber thereby forming cusps disposed longitudinally around an axis perpendicular to the plane of the workpiece. Or, the magnetic bucket may be formed by permanent magnets oriented so that the faces of the poles of each of the plurality of magnets face parallel with the interior of the processing chamber, with adjacent magnets having opposite poles facing each other thereby forming cusps disposed longitudinally around an axis perpendicular to the plane of the workpiece of the processing chamber. The present invention may employ open circuit magnets, closed circuit magnets, or a combination of the open and closed circuit magnets to confine the plasma away from the reactor components and to inhibit plasma from entering the vacuum pump and workpiece insertion openings. Furthermore, the magnetic bucket of the present invention may be formed by passing current through conductors located around the central axis of the processing chamber, or by a combination of conductors and permanent magnets.
Other embodiments of the present invention provide protection to reactor components and also provide precise control of the ion energy of the plasma generated by the present invention. One embodiment of the present invention provides an inner wall member. The inner wall member may be removably secured within the processing chamber so as to form an interior wall surface exposed to the plasma. The inner wall, therefore, may be easily replaced after being degraded by the plasma.
In a further embodiment, the present invention may provide the inner wall member as a controlled counterelectrode to improve workpiece processing. The voltage potential of all, or a portion of, the inner wall member may be controlled to provide a well defined anode for bias power that is applied to the workpiece pedestal for controlling ion energy. Typically, the inner wall member is grounded and insulated from the rest of the processing chamber. To further improve processing, the inner wall member may be temperature controlled so that process by-products, or process gases, do not condense on the inner wall and cause its impedance to drift.
The inner wall member of the above embodiments may be annular and attached to the top wall of the chamber. Additionally, it may have a cylindrical portion depending down so that the inner wall member may provide an anode to cathode ratio greater than, or equal to, about 3:1 to better control ion energy control so as to increase etch rate while not incurring additional charge damage.
The improved plasma reactor of the present invention may include all, or only some of the features of the above embodiments.